Human tankyrase belongs to the family of poly(ADP-ribose) polymerase (PARP) proteins which consists of 17 members that share a catalytic PARP domain. PARPs constitute a family of cell signaling enzymes present in eukaryotes which catalyze poly(ADP-ribosylation) (PARsylation) of DNA-binding proteins and other substrate proteins. PARPs are also known as poly(ADP-ribose) synthases or poly(ADP-ribose) transferases (pARTs). Some PARPs also transfer single ADP-ribosyl-moieties. These enzymes, for example, play an important role in the immediate cellular response to DNA damage. In response to DNA damage induced by ionizing radiation, oxidative stress and DNA-binding anti-tumor drugs, PARPs add ADP-ribose units to the carboxylate groups of aspartic and glutamic residues of target proteins. This poly(ADP-ribosylation) is a post-translational modification process that triggers the inactivation of the acceptor protein through the attachment of a complex branched by a polymer of ADP-ribose units. ADP ribosylation is a post-translational protein modification process in which the ADP-ribose moiety is transferred from NAD onto specific amino acid side chains of target proteins (Schreiber et al., 2006, Nature Reviews Cell Biology, 7: 517-528).
PARP family proteins are promising therapeutic targets. PARP1 and PARP2 play a role in DNA damage responses and PARP inhibitors sensitize cancer cells for drug and radiation therapies. In addition, PARP1 has been linked to other diseases including inflammation, neuronal cell death and ischemia. Tankyrases (TNKS1 and TNKS2), which share high sequence similarity with PARP1, are also emerging therapeutic targets. Tankyrases were initially known as regulators of telomerase activity and are involved in DNA damage responses and Wnt signaling (Wahlbert et al., 2012, Nat. Biotechnol., 30(3): 283-288).
The tankyrase protein family consists of tankyrase 1 (TNKS1) and tankyrase 2 (TNKS2) which share 85% amino acid identity. Biological functions of both tankyrase 1 and tankyrase 2 were studied in genetically engineered mice lacking mouse tankyrase 1 and/or tankyrase 2. Tankyrase 2-deficient mice developed normally and showed no detectable change in telomere length, but did show a significant decrease in total body weight that might reflect a role of tankyrase 2 in glucose or fat metabolism. No defects in telomere length maintenance were detected in tankyrase 1-deficient mice. However, in double-knockout mice lacking both tankyrase 1 and tankyrase 2 embryonic lethality was observed on embryonic day 10 (Chiang et al., 2008, PLoS One, 3(7): e2639).
A key feature of the Wnt/β-catenin pathway is the regulated proteolysis of the downstream effector β-catenin by the β-catenin destruction complex. The principal constituents of a β-catenin destruction complex are adenomatous polyposis coli (APC), axin and GSK3α/β. In the absence of Wnt pathway activation, cytosolic β-catenin is constitutively phosphorylated and targeted for degradation. Upon Wnt stimulation, a β-catenin destruction complex is dissociated, which leads to accumulation of β-catenin in the nucleus and transcription of Wnt pathway responsive genes.
It has been recently found that, in the Wnt/β-catenin pathway, a tankyrase inhibitor selectively inhibits the transcription mediated by β-catenin by promoting β-catenin degradation through stabilization of axin (Huang et al., 2009, Nature, 461(7264): 614-620).
Inappropriate activation of the pathway, mediated by overexpression of Wnt proteins or mutations affecting the components of the β-catenin destruction complex, thus leading to stabilization of β-catenin, has been observed in many cancers, for example, colon cancer, gastric cancer, hepatocellular carcinoma, breast cancer, medulloblastoma, melanoma, non-small cell lung cancer, pancreatic adenocarcinoma and prostate cancer (Waaler et al., 2012, Cancer Res., 72(11): 2822-2832). Notably, truncating mutations of a tumor suppressor APC are the most prevalent genetic alterations in colorectal carcinomas (Miyaki et al., 1994, Cancer Res., 54: 3011-3020). In addition, Axin1 and Axin2 mutations have been identified in patients with hepatocarcinomas and colorectal carcinomas (Taniguchi et al., 2002, Oncogene, 21: 4863-4871; Liu et al., 2000, Nat. Genet., 26: 146-147). These somatic mutations result in Wnt-independent stabilization of β-catenin and constitutive activation of 3-catenin-mediated transcription. Furthermore, deregulated Wnt pathway activity has also been implicated in many other non-cancer diseases including osteoporosis, osteoarthritis, polycystic kidney disease, pulmonary fibrosis, diabetes, schizophrenia, vascular diseases, cardiac diseases, non-oncogenic proliferative diseases, neurodegenerative diseases such as Alzheimer's disease, etc.
Therapeutics which are directed to and can correct dysregulation of the Wnt signaling pathway have been implicated in conditions such as bone density defects, coronary disease, late-onset Alzheimer's disease, familial exudative vitreoretinopathy, retinal angiogenesis, tetraamelia, Muellerian-duct regression and virilization, Serkal syndrome, type 2 diabetes, Fuhrmann syndrome, skeletal dysplasia, focal dermal hypoplasia and neural tube defects. Although the introduction has focused on the relevance of Wnt signaling in cancer, the Wnt signaling pathway is of fundamental importance in a broad range of human diseases, not necessarily being limited to the examples provided above for illustrative purposes.
Meanwhile, it has recently been reported that intracellular axin levels are influenced by poly(ADP-ribose) polymerase family members tankyrase-1 and tankyrase-2 (also known as PARP5a and PARP5b) (Nature Chemical Biology, 2009, 5: 100; Nature, 2009, 461: 614). The tankyrase enzymes are able to poly-ADP ribosylate (PARsylate) axin, which marks this protein for subsequent ubiquitination and proteasomal degradation. Thus, it would be expected that in the presence of an inhibitor of tankyrase catalytic activity, the axin protein concentration would be increased, resulting in higher concentration of the destruction complex, decreased concentration of unphosphorylated intracellular β-catenin and decreased Wnt signaling. An inhibitor of tankyrase-1 and -2 would also be expected to have an effect on other biological functions of tankyrase proteins (e.g., chromosome end (telomere) protection, insulin responsiveness and spindle assembly during mitosis) (Chang et al., 2005, Biochem. J., 391: 177-184; Chi et al., 2000, J. Biol. Chem., 275: 38437-38444; Bae et al., 2003, J. Biol. Chem., 278: 5195-5204).